Single Radio Voice Call Continuity SRVCC is a Long Term Evolution (LTE) functionality that allows a Voice over Internet Protocol (IP) (VoIP)/IP Multimedia Subsystem (IMS) call in the LTE packet domain to be transferred to a voice domain, e.g., Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), or Code Division Multiple Access (CDMA) lx. For an operator with a legacy cellular network who intends to deploy IMS/VoIP based voice services in conjunction with the rollout of the LTE network, Single Radio Voice Call Continuity SRVCC offers VoIP subscribers a coverage over a much larger area than would otherwise be available during rollout of the LTE network.
As shown in FIG. 1, a SRVCC-capable user terminal, here a user equipment UE, engaged in a voice call will determine that it moves away from the coverage area of a LTE network and inform the LTE network accordingly. The LTE network determines that the voice call needs to be moved to the legacy circuit domain and notifies a Mobile Switching Center (MSC) in the legacy circuit switched domain about the need to transfer the voice call from the packet switched domain to the legacy circuit switched domain. It is noted that “CS” denotes circuit switched and “PS” denotes packet switched.
As shown in FIG. 1, the MSC in the legacy circuit switched domain establishes a bearer path for the user terminal in the legacy circuit switched domain and notifies an IMS core that the call leg of the user terminal is moving from the packet switched domain to the legacy circuit switched domain. Also, the user terminal will switch its internal voice processing from VoIP to legacy circuit switched voice processing, and the voice call will continue.
As shown in FIG. 1, when the legacy circuit switched capable network also has an associated packet switching capability and supports concurrent circuit/packet switched operations, the subscriber's data service can as well be transferred to a packet switched domain of the legacy network in conjunction with the switching of the voice call from the packet switched domain to the circuit switched domain. In such a case when the voice call finishes and the user terminal re-enters LTE coverage, the packet switched data service can be re-transferred to the packet switched LTE network.
However, transfer of both circuit/packet switched services to the legacy network increases the likelihood of errors compared to the implementation of Single Radio Voice Call Continuity SRVCC alone. Reasons for this are increased signaling failure likelihood and handover complexity with respect to packet switched services. This is of particular relevance for specific type of calls, e.g., an IMS emergency call.